The pre-requisite to the cXML Purchase Request send capability is the cXML PurchOut Protocol Handler that you can find details here.
You can find the code here
For the purchase request send there are two projects. One is a project with just new objects and the second is a project with objects that you will have to merge as they touch existing AX objects.
The assumption is you know how to create a model file, label file and import XPO projects as that will be a pre-requisite for downloading this code.
The code was tested in AX2012 R3 CU8-CU11 and if you handle the merge could be imported in AX2012 R2.
For an overview of the new objects you can refer to this video.
For an overview of the merged objects you can refer to this video.
You can see this post for an overview of the changes you can see in the video for the merged objects
Refer to this post for an overview of the cXML Purchase Order Send.